1. Field of the Invention
The present invention relates to a frictional roller transmission.
2. Related Background Art
As a transmission for transmitting an engine power between shafts, a geared transmission is generally and widely used. In the geared transmission, an appropriate backlash is inevitably required for a smooth operation because of a configurational error, or the like, in manufacturing of the gear. However, when the geared transmission is used in a case when the transmission is to be frequently rotated forward and backward, the gear surface is impacted when the rotational direction is reversed due to the backlash, thereby generating a noise.
To cope with this inconvenience, there is conventionally known a frictional roller transmission, as power transmission means with no play (backlashless), which presses rollers to use a frictional force therebetween, thereby transmitting the power. For example, in a planetary roller reduction gear, planetary rollers which are disposed around the central roller and a ring roller inscribing the planetary rollers therein are assembled by shrinkage fit, or the like, with an interference so that a predetermined pressurizing power is generated between the rollers owing to elastic deformation of the respective rollers, thereby transmitting the engine power.
In a normal frictional roller transmission, a predetermined inter-roller pressing force is caused to work to generate a frictional force for power transmission. For this reason, the power transmission can be carried out only to the extent of the initially set pressing force, so that it is necessary to apply at the initial stage a very large pressing force which is corresponding to the maximum possible transmission torque.
However, in the normal frictional roller transmission, since it always has a very large pressing load applied thereon, an internal loss due to repeated elastic deformation upon rotation is generated irrespective of an actual transmission torque, and moreover, a loss due to an increase of a working torque of a bearing for receiving the load is also generated. As a result, there arises a problem that the working torque becomes very large, so that in an area in which a transmission torque is small, a loss is large and the efficiency in transmission becomes very poor.
According to Japanese Patent Application Laid-Open No. 6-135339, between a frictional roller on an input side and a frictional roller on an output side, there is interposed torque-restricting intermediate roller which transmits torque and is separated from the above both rollers when receiving torque having a predetermined magnitude or more. With this arrangement, when receiving the torque having the predetermined magnitude or more, the intermediate roller is separated from the both rollers so that an excessively large torque does not work on the respective rollers.
Further, according to Japanese Patent Application Laid-Open No. 6-288453, there is disclosed a torque cam transmission (perpendicular type rolling transmission device) as a transmission for working a pressing load in response to a transmission torque. This transmission device is effective for adjusting the pressing force in one-directional rotation, but has a defect that, if this transmission device is of a transmission direction reversible type, a large displacement of the roller for changing a cam surface is generated upon reversal of the rotational direction of the torque cam. As a result, there is a problem that a transmission delay or a colliding sound between the cam surface and the roller is produced.
Also, according to Japanese Patent Application Laid-Open No. 2000-16313 or Japanese Patent Application Laid-Open No. 2000-16314, there is disclosed a wedge roller transmission in which a ring roller and a sun roller are made slightly eccentric to each other in a planetary roller transmission to provide a planetary roller with a wedge effect, thereby obtaining a pressing power corresponding to the torque. However, this transmission has a problem that it can transmit the engine power only in one-directional rotation.